We have made the following observations about aging in the auditory system and auditory cortex. Measurement of the audiograms of rats indicate a progressive loss in sensitivity below 4 kilohertz and above 30 kilohertz after 25 to 30 months of age. In the cochlea ganglion cell, the loss is greatest in the basal (highest frequency) and apical (lowest frequency) ends. In the auditory nerve, there is age related axonal degeneration. In the auditory cortex between 3 and 36 months of age, many layer V pyramidal cells lose dendritic branches. During the period of dendritic loss, the mean area of the nucleus decreased and the mean length of the nuclear envelope increased. Biochemically, we have found that this is accompanied by changes in the physical properties of the protein of the nuclear matrix. Somewhat similar changes occur in the olfactory bulb. Here the number of presynaptic cells (olfactory receptors) must be compared with the number of synapses in olfactory glomeruli and the number and size of the postsynaptic mitrial cells to interpret the changes with age. In the choroid plexus, burrs develop on the surface of the mitrial cells with age. The burrs are removed by macrophages. Procedures have been developed for physically isolating the microcirculation of the brain, and characterizing its collagen, elastin and glycoproteins.